Calendered paper product and method of producing a calendered paper web

ABSTRACT

The invention relates to a method for producing a calendered paper web. According to the method, a paper web is formed from a fibrous raw material in a paper machine, and the paper web is calendered. According to the invention, a fibrous raw material is used which is at least partly made up of a chemimechanical pulp of a species of the  Populus  family, and the calendering is performed by online soft-calendering. By means of the invention, the gloss and smoothness of papers can be improved without decreasing their bulk.

The present invention relates to a method for producing a calenderedpaper web.

According to such a method a paper web is formed in the paper machinefrom a fibrous raw material and the web is calendered.

The invention also relates to a method for producing a coated andcalendered paper having a predetermined gloss and to a calendered paperproduct.

Calendering is a very important product treatment step in the productionof most paper grades. In calendering, the surface of the paper is evenedso that the surface becomes smooth, any variations in the thickness ofthe paper are evened out, and the paper becomes glossy in the desiredmanner. In calendering the printing properties of the paper areultimately brought to the level required for a printed product so that,for example, the gloss of the printed surface is as high as possible.

There are a number of calendering techniques. If the gloss of papers isabove approx. 40-50% (Hunter gloss, 75°), they are called glossy papers.The calendering process is in this case usually so-calledsupercalendering, although there are also other, less often used optionsfor, for example, boards. Respectively, if the gloss of papers is below40-50%, they are called matt, silk or satin papers. According to whetherglossy paper or matt paper is concerned, the surface material of thecalender rolls and the calender process conditions, above all the rolltemperatures and the nip pressure, but possibly also the calender speedand steaming, are set at different values. While with glossy paper theaim in principle is to achieve as high a gloss as possible, matt paperis above all desired to be very smooth, but so that the structure of thesurface will not reflect light in the manner of glossy paper.

There are two significant problems involved with calendering. First, awell-known disadvantage caused by calendering is that, as the glossand/or smoothness of the paper increases during calendering, thethickness and bulk of the paper are reduced considerably. A decrease inbulk is in practice always also associated with a decrease in theopacity and stiffness of the paper.

The other problem, significant in a supercalendering process implementedas a separate process step, is that the running speed of the calendersis slower than that of a modern paper machine. The design speeds of newprinting paper machines are currently in the order of up to 1800 m/min,whereas the speed of, for example, supercalenders has long been in theorder of 500-800 m/min.

Since the running speed of the supercalender has been lower than that ofa paper machine or a coating machine, it has been necessary to acquireseveral supercalenders for a paper mill for the after-treatment of theincreased production quantities of the actual paper making. Severalsolutions weakening the efficiency and the working conditions of thepaper mill have resulted: It has been necessary always to stop thecalender for the duration of roll replacements, which has resulted inloss of time and in the roll start offage. The extra process steprequires hoists, the use of which involves risks of occupational safety.An offline calender placed separately from the paper machine linerequires more space than if the same apparatus were placed in connectionwith the paper machine or the coating machine. The energy requirement inan offline calender is also higher, since the paper needs to bereheated. The lathe-turning of the supercalender rolls is a separatecost-inducing work step, which should preferably be entirely eliminated.Furthermore, since each supercalender requires a running crew for shiftwork, and if there are several supercalenders, this causes a significantcost to the mill.

The production capacity of a supercalender has in practice been limitedby the fact that it has not been possible to place simultaneously hightemperature loads and high pressure loads on rolls made of naturalmaterials. The risk has been damage to the rolls in the lowest roll nipsof supercalenders.

In order to avoid roll damage, the running method has in practice beenthat the upstream rolls have been run at high temperatures but lowpressures. Even though the paper web does thus become heated, owing tothe low pressure the transfer of heat is not the best possible. Whiletraveling through several roll nips the paper is gradually heated up andthereby becomes more formable. In the nips of the downstream end of thesupercalender it has respectively been possible to increase thepressure, but the limit has been the above-mentioned risk of rolldamage. The end result is that the paper is ultimately calendered ifthere are enough roll nips.

A running method such as this is, however, very inefficient, and theprocess running speed remains low. If the speed were increased, thepaper would not have time to heat up and would arrive too cold at theso-called bottom rolls. The result would be insufficient quality of thepaper.

The fact that the forming of paper gloss is in this manner indirectlydependent on the running speed of the supercalender also leads to anadditional problem. Since it has been necessary always to stop thesupercalender for the duration of roll replacement, the quality, inparticular the gloss, of paper varies during the acceleration andbraking of the supercalender. This results in waste paper and lostproduction time.

From the slow heating up of paper there also follows the disadvantagethat the entire paper (in the z direction) is heated, whereas in termsof calendering it would be optimal if only the surfaces were heated.Paper is better formable (the polymers present in the paper are betterformable) the warmer it is. The purpose is specifically to form thepaper surfaces and to avoid compression of the inner part of the paper,in order also to obtain bulk, opacity and stiffness in the paper.

Recently, the so-called soft-calendering technique has made progressowing to the development of roll materials. The end result is that atpresent it is possible to construct from large-diameter rolls calendernips at which the temperatures and pressures are, in terms of thecalendering of the product, such that the soft-calender can be placedeven directly in the paper machine line. The linear pressure of asoft-calender is typically above 200 kN/m and may be up to 450-600 kN/m,whereas in supercalendering it remains typically below 200 kN/m. Thequality of the final product has been sufficient, in particular inmatt-surfaced paper grades, but the production of sufficiently glossygrades in the category of glossy papers has not been quite successful.

The object of the present invention is to eliminate the problemsinvolved with the prior art and to provide a novel option for thesmoothing and glazing of paper.

The invention is based on the surprising observation that, when there isused in the base paper a chemimechanical pulp in which at least themajor proportion of the fibers are aspen fibers or corresponding woodfibers, it is possible by suitable calendering to achieve simultaneouslya high smoothness and a high gloss, and a considerably better opacity,bulk and stiffness than in reference papers. This technique solves thecalendering problem that has been associated with the production of bothmatt and glossy papers. In the invention there is thus used a fibrousraw material which is at least in part made up of a chemimechanical pulpof a wood species of the Populus family, and the calendering is carriedout by online soft-calendering. A coated paper web can be used forproducing papers having a gloss above 50% by performing the calenderingat a temperature of 120-170° C. and a linear pressure of 250-450 kN/m.Respectively, from the same paper web there is obtained paper having agloss below 50% if the calender rolls are not substantially heated andif the calendering is carried out at a linear pressure of 200-350 kN/m.

By means of the invention, there is obtained a calendered paper inwhich, in the mechanical pulp present in it, at least 20-40% by weightof the fibers are in the fiber size fraction of 28/48 mesh and at least20% by weight in the fiber size fraction of <200 mesh.

The invention provides considerable benefits. Thus the invention can beexploited in the calendering of both glossy papers and matt papers, butin practice the online calendering provides a clear improvementspecifically for the production of glossy papers. As is evident from theexamples presented below, by means of the invention it is possible toimprove the gloss and smoothness of papers without lowering their bulk.In fact, by the method according to the invention, a product glossierand smoother than commercial paper grades is obtained with a bulk atleast 5% higher. The benefits of the invention are manifest inparticular in the calendering of coated papers.

It has further been observed, surprisingly, that with coatingscontaining mainly gypsum as the pigment, the brightness and opacity ofpapers treated according to the invention are further improved.

According to the invention, one and the same paper web can be used forproducing both glossy paper grades and matt papers by varying theconditions of calendering.

In the following, the invention will be examined in more detail with thehelp of a detailed description and with reference to the annexeddrawings.

FIG. 1 depicts the gloss of eight different paper grades as a functionof smoothness,

FIG. 2 depicts the bulk of the same paper grades as a function ofsmoothness, and

FIG. 3 further depicts the bulk of the same paper grades as a functionof gloss.

It should be pointed out that, even though in many places in thefollowing description only aspen is mentioned as the raw material forthe chemimechanical pulp, the invention can, however, similarly beapplied to other wood species of the Populus family. In general, woodfrom, for example, the following wood species are suitable for use inthe invention: P. tremula, P. tremuloides, P. balsamea, P. balsamifera,P. trichocarpa, P. heterophylla, P. deltoides and P. grandidentata.Aspen (Finnish indigenous aspen, P. tremula; so-called Canadian aspen,P. tremuloides) and aspen species cross-bred from various parent aspens,so-called hybrid aspens (e.g. P. tremula×tremuloides, P.tremula×tremula, P. deltoides×trichocarpa, P. trichocarpa×deltoides, P.deltoides×nigra, P. maximowiczii×trichocarpa) and other species producedby gene technology, as well as the poplar, are regarded as especiallyadvantageous. From them it is possible to produce a chemimechanical pulphaving sufficiently good fiber properties and optical properties for usein the present invention.

Preferably a chemimechanical pulp having a suitable fiber distributionis used, of the fibers of which at least 30%, advantageously at least50%, and preferably at least 70% are derived from aspen, hybrid aspen orpoplar. According to an especially preferred embodiment, there is usedin the invention an aspen CTMP of the fibers of which at least 20% byweight are in the fiber size fraction of <200 mesh. Preferably there isused an aspen CTMP of the fibers of which 20-40% by weight, preferablyapprox. 25-35% by weight, are in the fiber size fraction of 28/48 meshand 20-40% by weight, preferably approx. 25-35% by weight, in the fibersize fraction of <200 mesh. By 28/48 mesh is meant in this case afraction which passes a wire having a mesh of 28, but which is retainedon a wire of 48 mesh. Such a fraction contains fibers which provide asuitable bulk and stiffness for a paper layer. The fiber size fractionwhich passes the densest wire (<200 mesh) for its part provides a highsurface smoothness. The pulp concerned can be produced in a manner knownper se by a chemimechanical process having several refining steps, forexample two steps, and thereafter reject classification and rejectrefining. The fiber size distribution is adjusted to the desired valueby the joint effect of these steps.

By chemimechanical pulp production is meant in the present invention aprocess comprising both a chemical and a mechanical defibration step.Chemimechanical processes include the CMP and CTMP processes; in the CMPprocess the wood raw material is refined under normal pressure, whereasin the CTMP process a pressure refiner pulp is prepared. The yield ofthe CMP process is in general lower (less than 90%) than that of theCTMP process, which is due to the fact that its chemicals dosage islarger. In both cases the treatment of the wood with chemicals isconventionally performed with sodium sulfite (sulfonation treatment), inwhich case hardwood can also be treated with sodium hydroxide. A typicalchemicals dosage in the CTMP process is in this case approx. 0-4% sodiumsulfite and 1-7% sodium hydroxide and the temperature is approx. 60-120°C. In the CMP process the chemicals dosage is 10-15% sodium sulfiteand/or 4-8% sodium hydroxide (dosages calculated from dry wood) and thetemperature is 130-160 and respectively 50-100° C.

In the chemimechanical process the chips may also be impregnated with analkaline peroxide solution (APMP process). The peroxide dosage is ingeneral 0.1-10% (of the weight of dry pulp), typically approx. 0.5-5%.An alkali, such as sodium hydroxide, is added in the same amount, i.e.approx. 1-10% by weight.

The raw material of the CTMP process may consist of only aspen or someother wood of the poplar family, but it is also possible to incorporateinto it other species, such as hardwood, e.g. birch, eucalyptus andmixed tropical hardwood, or softwood, such as spruce or pine. Accordingto one embodiment, a chemimechanical pulp is used which contains atleast 5% softwood fibers. In the invention it is possible to use, forexample, a chemimechanical pulp containing 70-100% aspen fibers and0-30% softwood fibers. The bulk, strength properties and stiffness ofthe pulp can be increased with softwood fibers, in particular sprucefibers. It is also possible by controlling the process parameters of theCTMP process to affect the bulk and stiffness of a pulp made up solelyof aspen or a similar raw material.

After defibration, the chemimechanical pulp is usually bleached with,for example, hydrogen peroxide in alkaline conditions to a brightness of70-88%.

To modify the properties of the initial material, an aspen pulp can,when so desired, be mixed with chemical pulp so that there is obtainedfor slushing an initial material which nevertheless contains asignificant amount (at least 30% by weight) of a chemimechanical pulp.The chemical pulp used is preferably a chemical softwood pulp theproportion of which is in this case 1-50% of the dry weight of thefibers. It is, however, possible to use chemimechanical aspen pulpalone.

The paper pulp is slushed in a manner known per se to a suitableconsistency (typically to a solids content of approx. 0.1-1%) and isspread on the wire, where it is formed into a paper or board web. It ispossible to add to the fiber slush a filler, such as calcium carbonate,in general in an amount of approx. 1-50% of the weight of the fibers.According to a preferred embodiment of the invention, the paper web isprovided with a coating prior to calendering. Coating pastes can be usedas single-coat pastes and as so-called pre-coat and surface-coat pastes.In general the coating mix according to the invention contains at leastone pigment or mixture of pigments10-100 parts by weight, at least onebinding agent 0.1-30 parts by weight, and other additives known perse1-10 parts by weight.

A typical composition of the pre-coat mix is as follows:

Coating pigment 100 parts by weight (e.g. coarse calcium carbonate)binder 1-20% of the weight of the pigment additives and auxiliary agents0.1-10% of the weight of the pigment water balance

Water is added to the pre-coat mix so that the dry solids content isgenerally 40-70%.

According to the invention, the composition of the surface-coat mix orsingle-coat mix is, for example, as follows:

coating pigment I 10-90 parts by weight (e.g. fine gypsum) coatingpigment II 0-90 parts by weight (e.g. fine kaolin) coating pigment III0-90 parts by weight (e.g. fine carbonate) pigment in total 100 parts byweight binding agent 1-20 parts by weight additives and auxiliary agents0.1-10 parts by weight water balance

Water is added to a coating mix such as this so that the dry solidscontent is typically 50-75%.

According to the invention it is possible to use in the coating mixespresented above pigments having an abrupt particle size distribution, inwhich case at maximum 35% of the pigment particles are smaller than 0.5μm, preferably at maximum 15% are smaller than 0.2 μm.

The invention is applicable to any pigment. Examples that can be citedof the pigments include precipitated calcium carbonate, ground calciumcarbonate, calcium sulfate, aluminum silicate, kaolin (hydrous aluminumsilicate), aluminum hydroxide, magnesium silicate, talc (hydrousmagnesium silicate), titanium dioxide and barium sulfate, and mixturesthereof. Synthetic pigments can also be used. Of the pigments mentionedabove, the main pigments are kaolin, calcium carbonate, precipitatedcalcium carbonate and gypsum, which in general constitute over 50% ofthe dry solids in the coating mix. Calcined kaolin, titanium dioxide,satin white, aluminum hydroxide, sodium silico-aluminate and plasticspigments are additional pigments, and their amounts are in general lessthan 25% of the dry solids in the mix. Special pigments that can becited include special-quality kaolins and calcium carbonates, as well asbarium sulfate and zinc oxide.

The invention is applied especially preferably to calcium carbonate,calcium sulfate, aluminum silicate and aluminum hydroxide, magnesiumsilicate, titanium dioxide and/or barium sulfate, as well as mixturesthereof, in which case especially preferably the principal pigment inthe pre-coat mixes is calcium carbonate or gypsum and in surface-coatmixes and single-coat mixes the principal pigment consists of mixturesof calcium carbonate or gypsum and kaolin.

As an example of a suitable coating composition can be mentioned a mixwhich contains:

precipitated calcium carbonate 40-90 parts and kaolin 10-60 parts orgypsum 10-60 parts and binder 1-20% of the pigment thickener 0.1-10% ofthe pigment

Advantageous results have been arrived at by coating a paper web with acoating composition in which at least 30% of the pigment is made up ofgypsum. It has been observed, surprisingly, that gypsum pigmentationgives the base paper according to the invention high brightness and highopacity. Especially preferably, gypsum pigment is used for coating abase paper made from an aspen CTMP which possibly contains at maximum20% softwood fibers and the brightness of which is at least 75%. In thiscase the ISO brightness of the web can easily be raised with gypsumpigments to at least 85% and the opacity to at least 90% when thegrammage is 90 g/m².

It is possible to use as binders in the coating composition any knownbinders generally used in paper production. Besides individual binders,it is also possible to use mixtures of binders. Examples of typicalbinders include synthetic latexes made up of polymers or copolymers ofethylenically unsaturated compounds, e.g. copolymers of thebutadiene-styrene type, which possibly also have a comonomer containinga carboxyl group, such as acrylic acid, itaconic acid or maleic acid,and polyvinyl acetate having comonomers that contain carboxyl groups.Together with the materials cited above, it is possible further to useas binders, for example, water-soluble polymers, starch, CMC,hydroxyethyl cellulose and polyvinyl alcohol.

Furthermore, it is possible to use in the coating compositionconventional additives and auxiliary agents, such as dispersants (e.g.sodium salt of polyacrylic acid), agents affecting the viscosity andwater retention of the mix (e.g. CMC, hydroxyethyl cellulose,polyacrylates, alginates, benzoate), so-called lubricants, hardenersused for improving water-resistance, optical auxiliary agents,anti-foaming agents, pH control agents, and preservatives. Examples oflubricants include sulfonated oils, esters, amines, calcium or ammoniumstearates; of agents improving water resistance, glyoxal; of opticalauxiliary agents, diaminostilbene disulfonic acid derivatives; ofanti-foamers, phosphate esters, silicones, alcohols, ethers, vegetableoils; of pH control agents, sodium hydroxide, ammonia; and finally ofpreservatives, formaldehyde, phenol, quaternary ammonium salts.

The coating mix can be applied to the material web in a manner known perse. The method according to the invention for coating paper and/or boardcan be carried out with a conventional coating apparatus, i.e. by bladecoating, or by film coating or JET application.

During the coating, a coating layer having a grammage of 5-30 g/m² isformed at least on one surface, preferably on both surfaces.

An uncoated web or a web coated in the manner described above isthereafter directed to online soft-calendering. By online calendering ismeant in this case calendering carried out in connection with the papermachine, without intermediate reeling of the paper.

By soft-calendering is meant calendering in which at least one of thetwo rolls forming a nip has a soft coating. The linear pressure in thecalendering is generally at least 200 kN/m and the speed of thecalendering is at least 800 m/min. The gloss of a paper or board productcan be affected significantly by the linear pressure and temperature ofcalendering. In general, glossy paper products are obtained whencalendering is carried out at a high linear pressure and a hightemperature (e.g. approx. 120-170° C.). The gloss of these products isover 50%. The paper web is calendered in this case in an online calenderhaving at least two nips formed between a hard roll and a soft roll. Thelinear pressure in the calendering of paper is, for example, approx.250-450 kN/m.

The temperature of the coated paper web arriving at the calender is,when paper making, calendering and calendering are in the same line, ingeneral approx. 50-60° C. at the beginning of the calendering. Accordingto another embodiment of the invention, the calender rolls are notsubstantially heated; the initial temperature of the paper web isexploited in this embodiment. This alternative is suitable for theproduction of matt papers, in which case a calendered paper web having agloss below 50% is produced. The paper web is in this case calendered ata linear pressure of, for example, 200-350 kN/m.

By means of the invention it is possible to produce coated andcalendered material webs having excellent printing properties, goodsmoothness, and high opacity and brightness. An especially preferredproduct is a coated offset paper in which high gloss and high opacityand bulk are combined. The grammage of the material web may be 50-450g/m². In general the grammage of the base paper is 30-250 g/m²,preferably 30-80 g/m². By coating a base paper of this type, which has agrammage of approx. 50-70 g/m², with 10-20 g of coating/m²/side and bycalendering the paper there is obtained a product having a grammage of70-110 g/m², a brightness of at least 90%, an opacity of at least 90%,and a surface roughness of at maximum 1.3 μm in glossy paper and atmaximum 2.8 μm in matt paper. The gloss obtained for glossy paper is upto 65% (Hunter 75).

The following non-restrictive examples illustrate the invention. Themeasuring results indicated in the examples for the paper propertieswere determined by the following standard methods:

-   Brightness: SCAN-P66-93 (D65/10°)-   Freeness, CSF: SCAN M 4:65-   Opacity: SCAN-P8:93 (C/2)-   Surface roughness: SCAN-P76:95-   Bendtsen roughness: SCAN-P21:67-   Gloss: Tappi T480 (75/) and T653 (20/)

EXAMPLE 1 Production of Aspen CTMP

Aspen CTMP was prepared by impregnating the chips with chemicals, byrefining the impregnated chips in two steps, and by bleaching the pulpwith peroxide.

The following conditions were complied with in the process:

Impregnation of Pulp:

In 2 steps, with peroxide and lye and DTPA (chelating of metals), inaddition to recycling of the filtrates, additionally both chemicals areadded in dosages of approx. 10-15 kg/tonne.

Refining:

1^(st) step pressurized 4-5 bar, pulp drainability (CSF) approx. 300-400ml

2^(nd) step open/1-2 bar, pulp drainability (CSF) approx. 150-180 ml,after screening the drainability value drops to the desired level, i.e.approx. 90-100 ml.

Bleaching:

In 2 steps (medium consistency and high consistency) with a small amountof water, peroxide and lye each approx. 30 kg/tonne of pulp, targetbrightness approx. 80.

Thus a pulp can be produced which has the following properties; in thisexample, 85% of the fibers were aspen and 15% were spruce.

Fressness, CSF 90 PFI shives, 0.05% Result of BauerMcNett fiberscreening: retained on 28 mesh 3.3% 28/48 31.9% 48/100 19.0% 100/20013.5% passed 200 mesh 32.3% grammage g/m² 64.2 density, kg/m³ 549 airresistance, Gurley, s 106 brightness % 77.5 light scattering coefficientm²/kg 58.0 tensile index, Nm/g 35.0 tear index, mN m²/g 3.3 internalbond strength, J/m² 135

EXAMPLE 2 Production of Base Paper

Base paper was produced in a production-scale test from the CTMPaccording to Example 1, as follows:

The base paper was produced from a mixture into which there were dosed:

-   -   25% broke derived from the normal production of the mill and        consisting of birch sulfate pulp, softwood sulfate pulp and PCC        filler    -   75% fresh pulp containing 50% softwood sulfate pulp refined to        the level of SR 25 and 50% aspen CTMP according to Example 1.        The aspen CTMP was not postre-fined separately at all at the        paper mill; the pulp underwent a very light refining treatment        in the so-called machine pulp refining. The machine pulp is made        up of softwood sulfate and aspen CTMP together.

In addition, PCC was added to the paper as a filler so that the totalfiller content (including the filler from the reject) in the machinereels ranged from 11.8 to 13.2%.

The paper machine wire speed was 895 m/min; the possible speed range forthis grammage and this paper formula in this machine could be 1100-1200m/min. The paper was calendered lightly in a machine calender.

Several machine reels of paper were produced for both tests; thegrammage in one test was approx. 65 g/m² and the grammage in the other55g/m². The most important quality values of the paper were:

-   -   grammage 65.6 g/m²    -   filler content 12.0%    -   bulk 1.65 kg/dm³    -   brightness (D65/10° light), top side of paper 95.2    -   brightness (D65/10° light), wire side of paper 94.8    -   opacity 89.6%    -   Bendtsen porosity 420 ml/min    -   Bendtsen roughness, top side of paper 306 ml/min    -   Bendtsen roughness, top side of paper 355 ml/min    -   internal bond strength 300 J/m²    -   tensile strength, machine direction of paper 4.1 kN/m    -   tensile strength, cross direction of paper 1.3 kN/m    -   tear strength, machine direction of paper 439 mN    -   tear strength, cross direction of paper 545 mN

EXAMPLE 3 Coating and Calendering of Glossy Paper

Next, base paper according to Example 2 was coated and calendered with apilot apparatus.

The coating formula was:

-   -   Opacarb A 40 (PCC) 60    -   Hydragloss 90 (clay) 40 parts    -   Styronal FX 8740 (styrene-butadiene latex) 13 parts    -   CMC Finnfix 10 0.9 parts    -   Blancophor PSF 1 part

The solids content of the coating paste was 66% and its pH was 8.5.

The coating was carried out by JET application at a speed of 1100 m/min.The target amount of coating was 13 g/m² on each side of the paper.

After the coating, the paper was calendered as follows:

-   -   Speed 900-1100 m/min    -   Linear pressure range 250-450 kN/m    -   Calendering temperature 120-160° C.    -   Nips: 2+2 hard/soft

Thus there was obtained paper having very good quality properties interms of heatset-offset printing. Table 1 compares paper according tothe invention with a competitor, at present the paper which is themarket leader, the grammage of each paper being 90 g/m². Thecompetitor's paper was produced using—probably—as the short-fibered pulpa chemical birch pulp or possibly a chemical pulp containing eucalyptus,acacia or so-called mixed hardwood pulp. The gloss and smoothnessindicated in the table are mean values calculated from the values of thetop side and the wire side of the paper.

TABLE 1 Magic Gloss Paper according to StoraEnso the invention Bulk,kg/dm³ 0.87 0.97 Smoothness, PPS10, μm 1.4 1.3 Gloss % (Hunter 75) 63 65Opacity % 92.1 94.1 Brightness % (D6510 measurement) 92.2 94.5 b*-tone−6.0 −4.1

The results of Table 1 are also presented graphically in FIGS. 1-3,which cover several tests on paper produced by the method of theinvention, according to how the process parameters of calendering werevaried. The base paper and the coating were produced in the same mannerin all of the tests.

As is evident from the results in the table above and the accompanyingfigures, the paper according to the invention is glossier and smootherbut, nevertheless, its bulk is more than 10% better than thecompetitor's bulk. It is essential to note that in Examples 1, 2 and 3the speed of the apparatus was always within the range of 895-1100m/min. In practice it is thus possible to implement a machine linewherein paper production, coating and calendering are in the sameproduction line and the speed of the entire line is, for example,1100-1200 m/min.

Opacity is especially notable in the results of Table 1. Paper producedby the method according to the invention is so much better with respectto opacity that the opacity achieved by the competitors with a grammage90 g/m² could by the method according to the invention be achievedalready with a paper of 74 g/m². This calculation is based on the use ofthe Kubelka-Munk theory.

EXAMPLE 4 Coating and Calendering of Matt Paper

Base paper according to Example 2 was next coated and calendered with apilot apparatus.

The coating formula was:

-   -   Opacarb A 60 (PCC) 80 parts    -   Suprawhite 80 (clay) 20 parts    -   Styronal FX 8740 (Styrene-butadiene latex) 13 parts    -   CMC Finnfix 10 0.7 parts    -   Stereocoll FD (synthetic thickener) 0.3 parts    -   Blancophor PSF 1 part    -   Dispersant 0.15 parts

The solids content of the coating paste was 65% and its pH was 8.5.

The coating was carried out by JET application at a speed of 1100 m/min.The target amount of coating was 13 g/m² on each side of the paper.

After the coating, the paper was calendered as follows:

-   -   Speed 900-1100 m/min    -   Linear pressure range 200-300 kN/m    -   Roll temperature 50° C.; in practice need not be heated, since        the paper web, coming from the paper machine, raises the        temperature to this range    -   Nips: 1 soft/soft

Thus a paper was obtained which had very good quality properties interms of heatset-offset printing. Table 2 compares the paper accordingto the invention with competitors, the grammage of all of the papersbeing 90 g/m². The papers of the competitors had been producedusing—probably—as the short-fibered pulp a chemical birch pulp orpossibly a chemical pulp containing eucalyptus, acacia or so-calledmixed hardwood pulps. The gloss and smoothness indicated in the tableare mean values calculated from the values of the top side and the wireside of the paper.

TABLE 2 KymPrint Paper G-Print UPM- Lumimatt according to StoraEnsoKymmene StoraEnso the invention Bulk, kg/dm³ 1 0.94 0.97 1.08Smoothness, PPS10, 3.6 2.85 2.9 2.5 μm Gloss % (Hunter 75) 15 24 27 20Brightness % (D6510 93.5 96.5 95.0 95.0 measurement) Opacity % 93.3 93.293.6 95.0 b*-tone −6.5 −19 −6.5 −4.5

The results of Table 2 are also presented graphically in FIGS. 1-3,which cover several tests on paper produced by the method of theinvention, according to how the process parameters of calendering werevaried. The base paper and coating were produced in the same manner inall of the tests.

The paper according to the invention is smoother but, nevertheless, itsbulk is on average over 10% better then the bulk of the bestcompetitors. In matt papers the gloss value is not as essential aquality value as the smoothness of the paper, but even with respect togloss, the paper according to the invention is within the same range asthe competitors.

Opacity is especially notable in the results of Table 2. The paperproduced by the method according to the invention is with respect toopacity so much better that the opacity achieved by the competitors witha grammage of 90 g/m² could be achieved with the paper producedaccording to the invention already with a 76 g/m² paper. Thiscalculation is based on the use of the Kubelka-Munk theory.

It is essential to note even in this example that in Examples 1, 2 and 4the speed of the apparatus was always within the range of 895-1100m/min. It is thus in practice possible to implement a machine linewherein paper production, coating and calendering are in the sameproduction line and the speed of the entire line is, for example,1100-1200 m/min.

1. A method for producing a calendered paper web, according to which method a fibrous raw material is formed into a paper web in the paper machine and the paper web is coated and the coated paper web is calendered, characterized in that a fibrous raw material is used, at least 30% by weight of which is made up of a chemimechanical pulp of a species of the Populus family, and the calendering is carried out at a linear pressure of approximately 250-450 kN/m, a temperature of approximately 120-170° C. and a speed of 800 m/min in an online calender, without intermediate reeling of the web, said calender having at least two nips formed between a hard roll and a soft roll, to produce a coated paper product having a gloss over 50%.
 2. The method according to claim 1, characterized in that a fibrous raw material is used which contains CTMP in which at minimum 30% of the fibers are derived from aspen, hybrid aspen, or poplar.
 3. The method according to claim 1 characterized in that an aspen CTMP is used in which at minimum 20% of the fibers are in the fiber size fraction of <200 mesh.
 4. The method according to claim 1, characterized in that an aspen CTMP is used in which 20-40% of the fibers are in the fiber size fraction of 28/48 mesh and 20-40% in the fiber size fraction of <200 mesh.
 5. The method according to claim 1 characterized in that a chemimechanical pulp is used which contains at minimum 50% aspen fibers.
 6. The method according to claim 1 characterized in that a chemimechanical pulp is used which contains 70-100% aspen fibers and 0-30% softwood fibers.
 7. The method according to claim 1 characterized in that a fibrous raw material is used which contains a mixture of chemimechanical pulp and chemical pulp, the proportion of the chemimechanical pulp being at minimum 30% of the dry weight of the fibers.
 8. The method according to claim 7, characterized in that the chemical pulp used is a softwood pulp the proportion of which is 5-50% of the dry solids weight of the fibers.
 9. The method according to claim 1 characterized in that the paper web is provided with a coating layer before calendering.
 10. The method according to claim 9, characterized in that the paper web is coated with a coating composition containing as a pigment a precipitated calcium carbonate, ground calcium carbonate, kaolin, gypsum, chalk and/or talc.
 11. The method according to claim 10, characterized in that the coating is carried out with a coating composition containing precipitated calcium carbonate 40-90 parts and kaolin 10-60 parts or gypsum 10-60 parts and binding agent 1-20% of the pigment thickener 0.1-10% of the pigment.


12. The method according to claim 10, characterized in that the paper web is coated with a coating composition in which at minimum 30% of the pigment is made up of gypsum.
 13. The method according to claim 12, characterized in that an aspen CTMP is used which contains at maximum 20% softwood fibers and the brightness of which is at minimum 70%, and the paper web is coated with a gypsum pigment in order to produce a paper web having a brightness of at minimum 80%.
 14. The method according to claim 9 characterized in that the coating is carried out by jet application.
 15. The method according to claim 1 characterized in that on at least one surface, of the paper web there is formed a coating layer having a grammage of 5-30 g/m².
 16. The method according to claim 1 characterized in that the calendering speed is at minimum 900 m/min.
 17. A method according to claim 1 for producing calendered paper having a predetermined gloss above 50%, wherein in the chemimechanical pulp at least 20% of the fibers are in the fiber size fraction of <200 mesh.
 18. Coated and calendered paper produced by a method according to claim 1, wherein the fibrous raw material comprises chemimechanical aspen pulp and 20-40% of the fibers are within the fiber size fraction of 28/48 mesh and 20-40% within the fiber size fraction of <200 mesh.
 19. The paper according to claim 18, characterized in that it is coated with a coating composition which contains a gypsum pigment.
 20. The paper according to claim 18 characterized in that the grammage of the paper is 50-350 g/m², the amount of coating is 10-40 g/one side of the paper, and the brightness of the paper is at minimum 80%.
 21. The paper according to claim 19, characterized in that the grammage of the paper is at maximum 100 g/m², the grammage of the base paper is 30-80 g/m² and the amount of coating is 5-20 g/m2, and the brightness is at minimum 92%. 